GB2380434A

GB2380434A - A method for estimating a change in the amount of oxidants stored in the catalyst of an exhaust system.

Info

Publication number: GB2380434A
Application number: GB0213528A
Authority: GB
Inventors: Donald James Lewis; Kevin Ronald Carlstrom; Jr Stanley Larue Bower; Stephen L Hahn
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2001-06-20
Filing date: 2002-06-13
Publication date: 2003-04-09
Anticipated expiration: 2022-06-13
Also published as: DE10227175A1; US6751947B2; DE10227175B4; GB0213528D0; US6453662B1; US20030005684A1; GB2380434B

Abstract

A method of estimating a change in the amount of oxidants stored in the catalyst of an exhaust system comprises determining an amount of oxidants available for being stored in a catalyst 52, estimating current amount of oxidants stored in the catalyst, determining the amount of oxidants required to oxidise by-products being produced by the engine at a given engine air/fuel ratio and then estimating an amount of oxidants which are retained or released by the catalyst over a period of time based on the amount of oxidants available for being stored and the amount of oxidants required to oxidise by-products being produced by the engine in that time period. The estimation of the amount of oxidants which are retained or released by the catalyst may be further based on a flow rate of oxidants in an exhaust manifold 48 of the exhaust system and a storage volume of the catalyst, on the temperature or degree of degradation of the catalyst. The by products may be reductants such as hydrocarbons. The estimated total of oxidants stored may be compared with a measured amount of oxidants stored. The exhaust system may comprise an exhaust gas sensor [EGO] 53 downstream of the catalyst. Also claimed is a system for deriving an air/fuel adjustment employing an oxidant set-point generator responsive to at least one engine operating parameter and a target oxidant level, a current oxidant level generator and an oxidant level/capacity controller. The current oxidant level generator may estimate an amount of oxidants which are retained or released by the catalyst over a period of time and the amount of oxidants required to oxidise by-products being produced by the engine in that time period.

Description

- 1 A SYSTEM AND METHOD FOR ESTIMATING

OXIDANT STORAGE OF A CATALYST

This invention relates to adjusting the air/fuel ratio 5 in the cylinders of an internal combustion engine to control automotive emissions and in particular to a method and system for estimating an amount of oxidants stored in system catalyst for use in controlling the engine air/fuel ratio.

10 To minimize the amount of emissions exhausted into the atmosphere, modern automotive vehicles generally include one or more catalytic converters, or emission control devices, in the exhaust system of the vehicle.

15 These emission control devices store oxygen and NOx (collectively, "oxidants") from the vehicle exhaust stream when the engine is operated with a relatively lean air/fuel ratio. On the other hand, when the engine is operated with a relatively rich air/fuel ratio, they release the stored so oxygen and NOx, which then react with the HC and CO produced by the engine. In this way, the emission of both NOx and hydrocarbons (HC and CO) into the atmosphere is minimized.

It is a disadvantage with conventional air-fuel ratio 25 control systems that these systems attempt to maintain the engine at stoichiometric or another desired air-fuel ratio.

This has the disadvantage that engine air-fuel control is decoupled from the state of oxidant storage of the 30 emission control device which is compensated for in a conventional system by air-fuel feedback.

To overcome disadvantages with prior approaches, the inventors have developed a method for controlling the engine 35 air-fuel ratio to maintain the oxidant level stored in the emission system at a desired setpoint level.

However, the inventors have further recognized that to implement such a system, an accurate method of determining the amount of oxidants stored in the emission control device should be used. In particular, known estimators for 5 determining oxidants retained in an emission control device (for example, see EP-B-0598, 917) ignore the reductant that is produced at the same time the oxidants are produced.

Further, such known estimates degrade with time since lo engine operating and catalyst performance vary.

It is an object of this invention to provide an improved method and system for estimating a change in an amount of oxidants stored in a catalyst of an exhaust system coupled to an internal combustion engine.

According to a first aspect of the invention there is provided a method of estimating a change in an amount of oxidants stored in a catalyst of an exhaust system coupled so to an internal combustion engine, comprising determining a first amount of oxidants available for being stored in the catalyst and a second amount of oxidants required to oxidize by-products being produced by the engine based on at least an engine air/fuel ratio and estimating a third amount of Is oxidants that are retained by or released from the catalyst over a period based on said first amount of oxidants available for being stored in the catalyst and said second amount of oxidants required to oxidize hydrocarbons being produced by the engine.

Said step of estimating said third amount of oxidants that are retained by or released from the catalyst may be further based on a flow rate of oxidants in an exhaust manifold and a storage volume of the catalyst.

Said step of estimating said third amount of oxidants that are retained by or released from the catalyst may be further based on a temperature of the catalyst.

5 Said step of estimating said third amount of oxidants that are retained by or released from the catalyst may be further based on a parameter indicative of a deterioration of the catalyst.

lo Said step of estimating said third amount of oxidants that are retained by or released from the catalyst may be further based on a parameter indicative of engine air mass flow. The by-products may be reductants.

The by-products may be hydrocarbons being produced by the engine and the method may further comprise of adjusting said estimate of said third amount of oxidants that are retained in or released from the catalyst based upon a so measured operating parameter.

Said measured operating parameter may be determined based on a comparison of an estimated total amount of oxidants stored in the catalyst and a measured amount of 25 oxidants stored in the catalyst.

Said measured operating parameter may include a parameter indicative of air mass flow.

so Adjusting said estimate of said third amount of oxidants that are retained in or released from the catalyst may further comprise of adjusting said estimate of said third amount to a relatively low amount if said measured amount of oxidants stored in the catalyst is relatively low and adjusting said estimate of said third amount to a relatively high amount if said measured amount of oxidants stored in the catalyst is relatively high.

- There may be an exhaust gas sensor coupled to the exhaust system downstream of the catalyst and the measured operating parameter may be the output from the sensor.

In which case, adjusting said estimate of said third amount of oxidants that are retained in or released from the catalyst may further comprise of setting said estimate of said third amount of oxidants to a relatively low value when lo said sensor output indicates a mixture rich of stoichiometric and setting said estimate of said third amount of oxidants to a relatively high value when said sensor output indicates a mixture lean of stoichiometric.

15 According to a second aspect of the invention there is provided a system for deriving an air/fuel adjustment control signal for an internal combustion engine coupled to an exhaust system having a catalyst, the system comprising an oxidant set point generator that is responsive to at so least one engine operating parameter and a set point location signal to determine a target oxidant level in the catalyst, a current oxidant level estimator that is responsive to at least one engine operating parameter and that estimates a current amount of oxidants stored in the 25 catalyst, said current oxidant level estimator selectively determining an amount of oxidants available for being stored in the catalyst and an amount of oxidants required to oxidize hydrocarbons being produced by the engine based on an engine air/fuel ratio and an oxidant level/capacity 30 controller that is responsive to said target oxidant level and said current amount of oxidants stored in the catalyst, and that determines an air/fuel adjustment control signal.

Said current oxidant level estimator may further 35 estimate a volume of oxidants that are adsorbed in or desorbed from the catalyst over a period of time based on said amount of oxidants available for being stored in the

1 1 À 5 catalyst and said amount of oxidants required to oxidize hydrocarbons being produced by the engine.

Said current oxidant level estimator may further 5 estimate said volume of oxidants that are adsorbed in or desorbed from the catalyst based on a flow rate of oxidants in an exhaust manifold and a volume of the catalyst.

Said current oxidant level estimator may further selectively estimate a volume of oxidants that are adsorbed in or desorbed from the catalyst based on a temperature of the catalyst.

Said current oxidant level estimator may further 5 estimate a volume of oxidants that are adsorbed in or Resorbed from the catalyst based on a parameter indicative of a deterioration of the catalyst.

Said current oxidant level estimator may further 20 estimate a volume of oxidants that are adsorbed in or desorbed from the catalyst based on a parameter indicative of engine air mass flow.

The invention will now be described by way of example with reference to the accompanying drawing of which: FIGURE 1 is an illustrative block diagram of an internal combustion engine according to a preferred embodiment of the invention; FIGURE 2 is a schematic diagram illustrating the major functions of a preferred embodiment of the invented system and method; 35 FIGURE 3 is a flowchart that illustrates a preferred embodiment of the available oxidant storage estimator algorithm of the present invention;

- 6 FIGURE 4 is a flowchart that illustrates a preferred embodiment of the oxidant set point location algorithm of the present invention; FIGURE 5 is a schematic diagram illustrating the operation of the oxidant set point generator algorithm of the present invention; lo FIGURE 6 is a flowchart that illustrates a preferred embodiment of the current oxidant level estimator algorithm of the present invention; FIGURE 7 is a schematic diagram illustrating the 5 operation of the oxidant level/capacity controller algorithm of the present invention; FIGURE 8A is a graph that illustrates the relationship between the temperature of a catalytic converter and a 20 variable "C1" that is used to estimate an amount of oxidants stored in the catalytic converter; FIGURE 8B is a graph that illustrates the relationship between the age of a catalytic converter and a variable, 25 "C2" that is used to estimate an amount of oxidants stored in the catalytic converter; FIGURE 8C is a graph that illustrates the relationship between engine mass air flow and a variable 30 "C3" that is used to estimate an amount of oxidants stored in the catalytic converter; FIGURE 9 is a schematic diagram of an exemplary catalytic converter comprising three internal bricks; and FIGURE 10 is a graph that illustrates the relationship between flange temperature and spark retard gain.

r - 7 Figure 1 illustrates an exemplary internal combustion engine according to a preferred embodiment of the invention.

s A fuel delivery system 11 of a conventional automotive internal combustion engine 13 is controlled by controller 15, such as an EEC or PCM. Engine 13 comprises fuel injectors 18, which are in fluid communication with fuel rail 22 to inject fuel into the cylinders (not shown) of 10 engine 13, and temperature sensor 132 for sensing temperature of engine 13. Fuel delivery system 11 has fuel rail 22, fuel rail pressure sensor 33 connected to fuel rail 22, fuel line 40 coupled to fuel rail 22 via coupling 41, fuel delivery system 42, which is housed within fuel tank 15 44, to selectively deliver fuel to fuel rail 22 via fuel line 40.

Engine 13 also comprises exhaust manifold 48 coupled to exhaust ports of the engine (not shown). Catalytic To converter 52 is coupled to exhaust manifold 48. In the preferred embodiment, catalytic converter 52 is a multiple brick catalyst. Figure 9 illustrates an exemplary multiple brick catalyst having three bricks, 52A, 52B, and 52C.

Oxygen sensors 902, 904, and 906, preferably being EGO, UEGO 25 or HEGO sensors, are positioned respectively behind bricks 52A, 52B, and 52C. Referring again to Figure 1, a first conventional exhaust gas oxygen (EGO) sensor 54 is positioned upstream of catalytic converter 52 in exhaust manifold 48. A second conventional exhaust gas oxygen (EGO) so sensor 53 is positioned downstream of catalytic converter 52 in exhaust manifold 48. EGO sensors 53 and 54 may comprise other known oxygen or air/fuel ratio sensors, such as HEGO or UEGO sensors. Engine 13 further comprises intake manifold 56 coupled to throttle body 58 having throttle 35 plate 60 therein. Intake manifold 56 is also coupled to vapour recovery system 70.

- 8 Vapour recovery system 70 comprises charcoal canister 72 coupled to fuel tank 44 via fuel tank connection line 74.

Vapour recovery system 70 also comprises vapour control valve 78 positioned in intake vapour line 76 between intake 5 manifold 56 and charcoal canister 72.

Controller 15 has CPU 114, random access memory 116 (RAM), computer storage medium 118 (ROM), having a computer readable code encoded therein, which is an electronically 0 programmable chip in this example, and input/output (I/O) bus 120. Controller 15 controls engine 13 by receiving various inputs through I/O bus 120, such as fuel pressure in fuel delivery system 11, as sensed by pressure sensor 33; relative exhaust air/fuel ratio as sensed by EGO sensor 54 and EGO sensor 53, temperature of engine 13 as sensed by temperature sensor 132, measurement of inducted mass airflow (MAF) from mass airflow sensor 158, speed of engine (RPM) from engine speed sensor 160, and various other sensors 156.

Controller 15 also creates various outputs through I/O bus To 120 to actuate the various components of the engine control system. Such components include fuel injectors 18, fuel delivery system 42, and vapour control valve 78. It should be noted that the fuel may comprise liquid fuel, in which case fuel delivery system 42 is an electronic fuel pump.

The fuel delivery control system 42, upon demand from engine 13 and under control of controller 15, pumps fuel from fuel tank 44 through fuel line 40, and into pressure fuel rail 22 for distribution to the fuel injectors during so conventional operation. Controller 15 controls fuel injectors 18 to maintain a desired air/fuel (A/F) ratio.

Referring now to the logical block diagram of Figure 2, a preferred embodiment of the invented method of and system 35 for controlling various engine parameters, including the air/fuel ratio in the engine cylinders, engine spark and air mass flow, is described.

- 9 Figure 2 illustrates an overview of the invented system and method.

5 Generally speaking the invention tries to adjust the engine air/fuel ratio in such a manner as to maintain the oxidants stored in the catalyst 52 at or near a target oxidant set point. The oxidant set point can be determined in a variety of ways depending on the objectives of the 10 engine control strategy. In a preferred embodiment of the invention, the oxidant set point is determined and adjusted dynamically in response to engine operating parameters.

The invention also tries to control the oxidant storage IS capacity of the catalyst 52 by controlling the catalyst temperature through adjusting engine operating parameters, such as engine spark and induction air mass flow (MAF).

Blocks 202 through 222 of Figure 2 identify the so following input variables to the invented system: air mass flow in the intake manifold (202); engine speed (204); vehicle speed (206); catalyst temperature (208) ; catalyst age (210); exhaust air/fuel ratio (212); oxidant levels behind each brick in a multi-brick catalyst 52 (214); spark 25 limits (216); throttle position (218); exhaust flange temperature (220); and mbt spark (minimum spark for best torque)(222). One skilled in the art will recognize that these system so inputs can be measured, either directly or indirectly, or mathematically estimated according to various methods known in the art. Blocks 224, 226, 228, 230, and 232 of Figure 2 represent the major algorithms of the invented system, according to a preferred embodiment.

Block 224 of Figure 2 signifies an oxidant set point generator algorithm. The oxidant set point generator is an

1 0 algorithm for establishing a desired (or "target") volume of oxidants to be stored in the catalyst 52 as a percentage of the oxidant storage capacity of the catalyst. The target volume of oxidants is also referred to herein as the 5 "oxidant set point." Generally, the oxidant set point is determined based on engine speed and load (which is inferred from air mass flow), vehicle speed, and other operating parameters. The oxidant set point signal (225), i.e., the output of the oxidant set point generator (224), is used by lo the invented system, and particularly by the oxidant level/capacity controller (block 232) to control engine operation. A more detailed description of the algorithm

employed by the oxidant storage set point generator (224) is provided below in connection with a description of Figure 5.

Block 226 of Figure 2 signifies an "available oxidant storage estimator" algorithm. The available oxidant storage estimator algorithm (226) estimates the amount of oxidant storage capacity that is available in a catalyst so brick. This algorithm is implemented for each brick in a multiple-brick catalyst 52. The available oxidant storage of each brick is estimated based on the catalyst temperature (208) and the catalyst age (210). The estimated available oxidant signal (227) is provided to a "current oxidant level 25 estimator" (block 230) and the oxidant level/capacity controller (232). more detailed description of the

available oxidant storage estimator (226) is provided below in connection with the discussion of Figure 3.

so Block 228 signifies a "set point location" algorithm, which, in connection with a system having a multiple-brick catalyst 52, determines which of the bricks in the catalyst 52 is the "key brick." The key brick is that brick in the catalyst 52 upon which the system bases its engine control 35 strategy. In other words, the invented system attempts to control the engine operation to maintain a particular oxidant level at the key brick. The key brick changes from

time to time based on various engine operating conditions.

The set point location algorithm (228) determines the key brick based on the catalyst temperature (208), the catalyst age (210), and the available oxidant storage in each brick s (signal 227). The output signal of the set point location algorithm (229), i.e., the key brick location, is used by the oxidant storage set point generator (block 224) to determine the oxidant set point value (signal 225). A more detailed description of the set point location algorithm

lo (228) is provided below in connection with the discussion of Figure 4.

Block 230 of Figure 2 signifies a "current oxidant level estimator" algorithm, which estimates the 15 instantaneous oxidant level in a catalyst brick. In a system using a multiple brick catalyst 52, the current oxidant level estimator algorithm is implemented for each brick. The oxidant level in each brick is estimated based on the air mass flow (202), the catalyst temperature (208), so the exhaust air/fuel ratio (212) , and the estimate of available oxidant storage capacity in each of the bricks (227). The estimated amount of oxidants stored in each brick (signal 231) is provided to the oxidant level/capacity controller (232). A more detailed description of the

25 current oxidant level estimator algorithm (230) is provided below in connection with the discussion of Figure 6.

Block 232 signifies an "oxidant level/capacity controller", which calculates engine control signals 30 intended to cause the engine 13 to function so as to control the oxidant level in the catalyst 52 close to the oxidant set point, as well as to control the oxidant storage capacity of the catalyst 52. Specifically, the oxidant level/capacity controller (232) calculates an air/fuel 35 control bias signal (238) that is used to adjust the air/fuel ratio provided to the engine cylinders. The air/fuel control bias signal (238) is the primary mechanism

of adjusting the oxidant level in the catalyst 52. The oxidant level/capacity controller (232) also calculates an air mass bias signal (236) and a delta spark signal (234).

The air mass bias and delta spark signals are used to adjust 5 the oxidant storage capacity of the catalyst 52 by controlling the temperature of the catalyst. The oxidant level/capacity controller (232) further calculates Reset/Adaptive Coefficients, which essentially cause the oxidant level prediction algorithms to be reset or adjusted based on feedback signals. A more detailed description of

the oxidant level/capacity controller (232) is provided below in connection with a discussion of Figure 7.

Referring now to Figure 3, a more detailed description

15 of the "available oxidant storage estimator" algorithm (226) is provided. The available oxidant storage estimator (226) determines the total oxidant storage capacity that is available in a single brick of catalyst 52. It is desirable to make this calculation for each brick in the catalyst 52 20 to facilitate the determination of the desired oxidant set point, or oxidant target, in block 224 of Figure 2.

Therefore, for multiple-brick catalysts 52, the available oxidant storage estimator (226) is applied to each brick.

25 The available oxidant storage capacity in each brick is a function of the wash coat used on the catalyst 52, the temperature of the brick (208), and the deterioration of the brick (210). The wash coat factor, which depends upon the adsorption characteristics of the particular wash coat used so on the catalyst 52, is measured in grams per cubic inch and is a constant parameter for a given catalyst. The wash coat parameter can be pre-programmed into the algorithm at the time of manufacture. One skilled in the art will recognize that the temperature of each brick can either be measured 35 using conventional temperature sensors or estimated using various mathematical models.

Finally, the extent of catalyst deterioration can also be determined in a variety of ways. In the preferred embodiment of the invention, the extent of catalyst deterioration is inferred based on the current oxidant 5 storage capacity of the catalyst. A first preferred method for doing so is disclosed in U.S. Patent No. 5,848,528, which is hereby incorporated by reference. In summary,

first, the catalyst is filled with oxidants by running the engine with a lean air/fuel ratio for an extended period of lo time. After the catalyst is filled, the air/fuel ratio provided to the engine is made rich. The pre-catalyst oxygen sensor 54 detects the rich air/fuel condition in the exhaust almost immediately. However, because the HC and CO produced by the rich engine air/fuel ratio reacts with the 15 stored oxidants in the catalyst, there is a time delay until the post-catalyst oxygen sensor 53 detects a rich air/fuel ratio in the downstream exhaust. The length of the time delay is indicative of the oxidant storage capacity of the catalyst. Based upon the measured time delay, a co deterioration factor between O and 1 (O representing total deterioration and 1 representing no deterioration) is calculated. Alternatively, the method could be used in reverse, i.e., the catalyst could be depleted due to extended rich operation, after which the air/fuel ratio Is would be switched to lean operation. Similar to the original method, the length of the time delay until the post-catalyst sensor 53 registered a change in state would be indicative of the catalyst deterioration.

So A second preferred method of estimating the deterioration level of the catalyst uses the estimated current oxidant storage of the catalyst, as derived by the oxidant estimator model (described below in connection with Figure 6), to predict the level of deterioration of the as catalyst. Specifically, as described above, the engine controller 15 receives feedback signals from downstream EGO sensor 53. As is known in the art, when the output signal

of an EGO sensor switches from indicating a lean air/fuel condition in the exhaust stream to a rich air/fuel condition (or visa versa), this is an indication of emission breakthrough. In the case of a switch from rich to lean, 5 this is an indication that the oxidant content in the exhaust stream downstream of the catalyst is high, which means that the catalytic converter 52 has reached its capacity in terms of adsorbing oxidants. When this occurs, the oxidant estimator model (described in connection with lo Figure 6) is used to estimate the current volume of oxidants stored in the catalytic converter 52. From this estimate of the current oxidant storage volume, the system controller 15 can determine the level and rate of catalyst deterioration in a variety of ways. For example, the controller 15 can compare the current catalytic capacity to previous estimated catalytic capacities to determine the rate of catalyst deterioration. Further, the controller can determine that the catalyst has expended its useful life at the time when the oxidant storage capacity of the catalyst declines to a So pre-determined value.

Returning to Figure 3, block 302 signifies the start of the available oxidant storage estimator algorithm. (226) Blocks 20 3 and 210 illustrate that the individual brick 25 temperatures (208) and the catalyst deterioration factor (210) are dynamic inputs to the algorithm (226). The individual brick temperatures (208) are preferably measured with temperature sensors, and alternative preferred methods for determining the catalyst deterioration factor are 30 described above. At block 310, the theoretical maximum oxidant storage capacity of a catalyst brick during normal operating temperature is calculated. The maximum oxidant storage capacity, being a function of washcoat, is measured at a given temperature. This capacity is then multiplied by as the deterioration factor to produce a theoretical maximum oxidant storage.

However, if the current operating temperature is not normal, as during initial start-up conditions, then the current storage capacity of the brick may be less than its theoretical maximum value. Accordingly, the next step, at s block 314, is to estimate the current oxidant storage capacity of the brick based on the theoretical maximum storage capacity and the current temperature of the brick.

The estimated current oxidant storage capacity is a function of the maximum oxidant storage capacity and the catalyst lo temperature. The estimated current storage capacity of each brick (in grams per cubic inch) is the final output (227) of the available oxidant storage estimator (226), and it is used as input to each of the other main algorithms described in this invention. The available oxidant storage estimator IS algorithm is stopped at block 318.

Referring now to Figure 4, a more detailed description

of the oxidant set point location algorithm (228) will be described. An object of the oxidant set point location 20 algorithm (228) is to identify the particular brick in a multiple-brick catalyst 52 at which it is desirable to control the oxidant storage, i.e. the "set point location. "

Actually, the oxidant set point is positioned just behind a given brick. In this way, the available oxidant storage as capacity of the catalyst is considered to be that of the set point brick plus all of the bricks forward of the set point brick in the catalyst. Because the bricks in a catalyst tend to fill with oxidants unevenly, normally from front to back, and because oxidant storage is largely a function of so temperature, and because the storage capacity of catalyst bricks deteriorate over time, it is desirable to selectively choose where in the catalyst (i.e., which brick) to control the oxidant level around. Further, selectively choosing the key brick enables the system to better control the as distribution of oxidant storage throughout the various bricks in the catalyst.

- 16 At block 402 in Figure 4, the algorithm is started.

Blocks 208 and 210 signify the individual brick temperatures and the catalyst deterioration factor, respectively, as inputs to the algorithm. The catalyst deterioration factor 5 is determined according to one of the preferred methods described above. The individual brick temperatures (208) , and the catalyst deterioration factor (210) are used subsequently in the set point location algorithm to determine the oxidant set point location.

In block 405, a required oxidant reserve capacity is calculated for the entire catalyst. The oxidant reserve capacity is the current storage capacity of the bricks positioned behind the oxidant set point. It is desirable to 15 maintain a certain minimum oxidant reserve capacity to accommodate inaccuracies and transients in the system.

The oxidant capacity reserve is maintained so that if an unexpected rich/lean break occurs at the set point, so there is sufficient oxidant storage capability remaining in the catalyst (in the bricks positioned behind the set point) to prevent total system breakthrough. The catalyst reserve capacity is calculated from the amount of oxidant storage available in each brick (227), as well as induction air mass 25 (202),engine speed (204), vehicle speed (206), and catalyst brick temperature (208), as shown in block 407.

Specifically, the catalyst capacity reserve equals the total oxidant storage capacity of the catalyst less the so oxidant storage capacity in the bricks in front of the set point location. Because the engine control strategy focuses on controlling the air/fuel ratio based on the storage capacity of the bricks in front of the set point, any additional storage capacity of bricks located behind the set 35 point (as a result of the temperature of subsequent bricks rising) constitutes the available capacity reserve. As described below, the preferred embodiment of the invention

- 17 always maintains a certain storage capacity reserve by only adjusting the set point location if the resulting storage capacity reserve is greater than a certain minimum "required reserve". Based on the individual brick temperatures (208), the catalyst deterioration factor (210) and the required oxidant storage reserve (405), the oxidant set point location algorithm (228) determines the set point location according lo to blocks 406-418 and per the following description.

Initially, it is assumed that the set point location is the most forward brick (brick(1)) in the catalyst 52. That is, the invented system will control the engine air/fuel ratio based on the oxidant storage capacity of the first brick 15 only (which is the only brick located in front of the set point). At block 406, it is determined if (i) the temperature of the second brick (brick(2)) in the catalyst 52 exceeds a predetermined minimum brick temperature or (ii) if the deterioration factor of the first brick (brick(l)) is 20 greater than a predetermined maximum deterioration factor.

If either of these conditions is true, and if the oxidant storage capacity reserve of the catalyst with the set point being the second brick (brick(2)) is greater than the required reserve, then the set point location moves from the 25 first brick (brick(1)) to the second brick (brick(2)). If not, then the set point location remains at the first brick (brick(1)), as shown at block 408.

At block 410, a similar test is performed. It is 30 determined if the temperature of the third brick (brick(3)) is greater than a predetermined minimum temperature or if the deterioration factor of the second brick (brick(2)) is greater than a predetermined maximum deterioration factor.

If either of these conditions is true, and if the oxidant 35 storage capacity reserve of the catalyst would be greater than the required reserve with the third brick being the set point, then the set point location moves from the second

- 18 brick (brick(2)) to the third brick (brick(3)). If not, then the set point location remains at the second brick (brick(2)), as shown at block 412. Thus, the invented system controls the engine air/fuel ratio based on the s oxidant storage capacity of the first and second bricks together. This same procedure is repeated, as shown in blocks 414-418 until a final set point location is determined. One 10 skilled in the art will appreciate that the described oxidant set point location algorithm generally causes the set point to move from the forward bricks toward the rearward bricks as the temperature of the catalyst bricks increase from front to rear. This is because the storage 15 capacity of catalyst bricks increases with brick temperature. Thus, during a cold start, the oxidant set point location will usually start out being the first (most forward) brick in the catalyst, and the set point location will migrate rearward as the temperature of the rearward so bricks increase. Further, aging/deterioration of the catalyst will tend to move the oxidant set point location rearward in the chain of bricks more quickly, since the forward bricks will tend to have less capacity as they deteriorate. Finally, extended idle or low load (low air Is mass flow) operation of the vehicle may cause the set point location to migrate forward in the chain of bricks if the temperature of the rearward bricks falls. In general, it is desirable in the preferred embodiment of the invention to maintain the set point location at approximately one half to so two thirds of the total catalyst storage capacity to provide a preferred reserve capacity capable of sufficiently accommodating system transient inaccuracies.

The preferred embodiment of the oxidant set point as location algorithm described above involves identifying a particular brick as the set point. However, in an alternative preferred embodiment of the invention, the

- 19 oxidant set point can be established within any of the bricks of a multiple-brick catalyst. Thus, instead of setting the set point behind brick 1 or brick 2, for instance, the set point can be set at various points inside 5 of brick 1 or brick 2. The set point can then be moved through the interiors of the various bricks based on a calculation of the oxidant storage capacity before and after the set point within the brick. Using a model wherein the oxidant set point can be set inside of the various bricks lo may increase accuracy of the estimations and control of the oxidant storage.

Referring now to Figure 5, a more detailed description of the oxidant set point generator (block 224 in

15 Figure 2) is provided. An object of the oxidant set point generator (224) is to calculate a desired target oxidant storage amount, that is to say the oxidant set point that the system will attempt to maintain stored in the bricks in front of the set point location.

As indicated previously, the following input parameters are provided to the oxidant set point generator: (i) air mass (202); (ii) engine speed (204); 25 (iii) vehicle speed (206); (iv) available oxidant storage in each brick (227); (v) set point location (229); and (vi) throttle position (218).

so Based on these input parameters, the oxidant set point generator calculates a desired target oxidant storage level (225 of Figure 2) as a percentage of the total oxidant storage capacity of the catalyst 52. This desired target oxidant storage level (225), or "oxidant set point", is the 35 critical value upon which the engine control signals are generated.

In a preferred embodiment of the invention, as shown in block 504, the air mass (202), engine speed (204) and vehicle speed (206) parameters are used as index values into a three-dimensional look-up table (504). The output of the 5 look-up table (504) is a value that represents a desired percentage of available oxidant storage capacity in the catalyst 52. The values in the lookup table (502) are empirically determined based on optimal catalyst conversion efficiency, and they are preset at the time of manufacture.

lo Steady state efficiencies are used as a basis for determining desired oxidant set points, and set points that provide the highest efficiencies with some immunity to disturbances are selected. At block 506, a value indicative of the volume of available oxidant storage in the bricks in 15 front of the oxidant set point location in the catalyst is determined based on the set point location (229) and the available oxidant storage per brick (227). To do so, the desired percentage of available oxidant storage in the catalyst 52 (from 504) is multiplied by the volume of so available oxidant storage in the bricks in front of the set point (from 506) at block 512. The resulting product is a base oxidant set point, which consists of a target amount of oxidants to be stored in the catalyst 52.

25 A set point modulation function (508) is applied to the product at block 514 based on engine speed (204) and load (202) to improve catalyst efficiency, as is known by those skilled in the art. Finally, at block 510, a look-ahead multiplier value is determined based upon air mass (202) , so engine speed (204), vehicle speed (206) and throttle position (218) parameters. A purpose of the look-ahead multiplier is to adjust the oxidant set point based on expected future operating conditions. For example, the oxidant set point may be established at a relatively low 35 value after the vehicle operator tips out and the vehicle stops because it is reasonably certain that a tip-in condition will occur shortly thereafter. The expected tip

in condition will produce higher levels of NOX, and the low set point will compensate for this condition. The look-

ahead multiplier is applied at block 516 by multiplying the look-ahead multiplier by the modulated base set point. The 5 product is a final oxidant set point (225), representing a target oxidant storage level in the catalyst (in grams per cubic inch).

An alternative embodiment of the oxidant set point lo generator (224) involves using a four dimensional look-up table to combine the functions of the three dimensional look-up table (504) and the look-ahead multiplier determination (510). Essentially, the function of the look-

ahead multiplier would be incorporated into the fourth 15 dimension of the look-up table. In this embodiment, the oxidant set point would be determined from the four dimensional look-up table based on air mass (202) , engine speed (204), vehicle speed (206), and throttle position (218). The output of the four-dimensional look-up table so would be the target oxidant set point and no modification based on a look-ahead multiplier would be necessary.

In preferred embodiments of the invention, the oxidant set point is prevented from being set at a level that 25 exceeds the functional limits of the catalytic converter, i.e., greater than the total oxidant storage capacity of the catalyst or less than zero. Preferably, the oxidant set point is limited to between about 30% and about 70% of the total catalyst storage capacity. In other preferred JO embodiments of the invention, parameters other than engine speed and load and vehicle speed, such as catalyst temperature, EGR and ignition timing, may be used to determine a desirable oxidant set point. Moreover, the present invention is equally applicable to systems wherein as the oxidant set point is a constant value, such as, for example, 50i of the total oxidant storage capacity of the catalytic converter 52, in which case the entire oxidant set

- 22 point generator algorithm (224) could be replaced with a constant value.

Referring now to Figure 6, a more detailed description

5 of the "oxidant level estimator" algorithm (230), which estimates the instantaneous oxidant levels in the bricks of catalyst 52, is provided. The results of this algorithm are used ultimately by the oxidant level/capacity controller (232) to adjust the engine air/fuel ratio based on a 0 comparison of the estimated oxidant storage in the catalyst with the oxidant set point.

The oxidant level estimator algorithm begins at block 602. At block 604, it is determined whether an oxidant 15 state initialization is required, i.e., whether or not the vehicle has just been started. If the vehicle has just been started, then the oxidant estimator model must be initialized because oxidants tend to gradually fill the catalyst for a period after the vehicle has been turned off, so then are released as the catalyst cools. An initialization of the oxidant estimator model involves determining the oxidant state of the catalyst 52 based on the "soak time" (time since the vehicle was turned off) and the current temperature of the catalyst. If the soak time is relatively as long, then the current oxidant level of the catalyst 52 is determined to be a preset value corresponding to a "cold start" of the vehicle because it is assumed that the catalyst has filled with oxidant to a predictable level. On the other hand, if the soak time is relatively short, then 30 catalyst 52 has likely not yet filled with oxidant to the same extent as during an extended soak. Therefore, the initial oxidant state of catalyst 52 is determined based on the last oxidant state (before the vehicle was turned off), the soak time, the current catalyst temperature, and an 35 empirical time constant, as shown in block 610.

l - 23 Regardless of the initial oxidant level in the catalyst bricks, the current oxidant levels are calculated according to the oxidant level predictor model, or "observer", described below based on air mass (202), catalyst 5 temperature (208), exhaust air/fuel ratio (212), available oxidant storage (227) and reset and adaptive feedback parameters (240) derived from the oxidant level controller (232). The oxidant predictor model calculation occurs at block 608 according to the following method.

The actual amount of oxidants stored in the catalytic converter 52 is continually estimated using a mathematical oxidant predictor model or "observer." At preset times T. the oxidant predictor model estimates the amount of oxidants 15 (602) adsorbed and/or desorbed in the catalytic converter 52 over the time interval AT from the previous time Ti-1 to the current preset time Ti. A running total value is maintained in the RAM memory 116 that represents the current estimate of the amount of oxidants stored in the catalytic converter so 52. The estimated change in the amount of oxidants ( 02) stored in the catalytic converter is added to or subtracted from the running total value maintained in RAM 116 on an iterative basis. Therefore, at any one time, RAM memory 116 contains the most current estimate of the total amount of 25 oxidants stored in the catalytic converter 52.

Details of how a preferred embodiment of the oxidant predictor model estimates the amount of oxidants adsorbed/desorbed at the various preset times Ti (block 608) 30 will now be described.

First, the current air/fuel ratio provided to the engine cylinders is used to determine the amount of oxidants (02) that is either available for storage in the catalytic 35 converter 52 (as a result of lean air/fuel operation) or that is needed for oxidation of hydrocarbons (as a result of

- 24 rich air/fuel operation), according to the following equation: O2 = A (1 - A)* (1 + Yips)] * 32 ( 1) In Equation 1 above, one skilled in the art will recognize that the variable y represents a value that varies depending upon the type of fuel used in the system. For a lo normal gasoline engine, y equals 1.85. The variable represents the air/fuel ratio in the exhaust manifold 48 upstream of the catalytic converter 52. In the preferred embodiment of the invention, the variable is assigned the air/fuel ratio that is commanded by the controller 15 to be provided to the engine cylinders at a given time T. It is also possible to use the output of upstream EGO sensor 54 (in Figure 1) as the value for in Equation 1. Finally, the factor A represents the mole flow rate of air in the exhaust manifold 48, which is calculated according to the 2 0 following Equation 2: (1 +) (MWO2 + INS) ( 2)

In Equation 2, the variable y is again a value that varies with the type of fuel used in the system, which is 1.85 for gasoline. The mole weight of oxidant (MWo2) is 32 and the mole weight of nitrogen (MWn2) is 28. Accordingly, so for a gasoline engine, the factor A equals 0.00498 grams/sec. When Equation 1 is solved, a negative value for O2 indicates that oxidant is being adsorbed by the catalyst 52, and a positive value for O2 indicates that oxidant is being desorbed by the catalyst 52 to react with 35 hydrocarbons.

Once the amount of oxidants either available for storage in the catalytic converter or required for oxidation of the hydrocarbons being produced by the engine is determined, the next step is to estimate the volume of 5 oxidants that are actually adsorbed/desorbed by the catalytic converter. In the preferred embodiment, this estimation depends on several factors, including the volume of the catalytic converter 52, the flow rate of oxidants in the exhaust manifold 48, the percentage of the catalytic 0 converter that is already full of oxidants, and other physical and operational characteristics of the catalytic converter. According to the preferred embodiment of the present invention, the change in the amount of oxidants stored in the catalytic converter 52 between two preset times (aT) is estimated based on the following model: 60 2 = C * C2 * C3 * C4 IRK a * (1 _ Stored O 2) * ( O 2 Flow Rate) a' C V I l Max O2 Base Value for Oxygen being adsorbed 20 (3a) TO C *C *C *CONK *(StoredO2) *<o2FlowRate) 2*c tV 1* T L Max O2 Base Value 25 for Oxygen being desorbed (3b) As indicated above, Equation (3a) is used to calculate the change in oxidant storage in the catalytic converter if so the catalyst is in an adsorption mode and Equation (3b) is used if the catalyst is in a desorbtion mode.

In Equations (3a) and (3b), the variables C1, C2, and C3 are assigned values to compensate for various functional and 35 operational characteristics of the catalytic converter. The value of C1 is determined according to a mathematical function or look-up table based on the catalyst temperature.

- 26 The preferred embodiment of the invention uses a mathematical function represented by the graph in Figure 8A, which illustrates that a catalytic converter is most active 5 when the catalyst is hot and least active when it is cold.

The catalyst temperature can be determined according to several different methods that are well-known to those of skill in the art, including by a catalyst temperature sensor. After determined, the catalyst temperature is used lo to assign a value to C1 according to the function shown in Figure 8A.

The value of C2 in Equations (3a) and (3b) is determined based on the deterioration of the catalytic converter. The deterioration of the catalytic converter can be determined by a variety of well-known methods, including, for example, inferring such age or deterioration from the vehicle's total mileage (recorded by the vehicle's odometer) or total amount of fuel used over the vehicle's lifetime. Further, a so catalytic deterioration factor can be calculated according to one of the preferred methods described hereinabove.

Figure 8B shows a graphical representation of a preferred mathematical function used to assign values to C2 in the preferred embodiment of the invention. Figure 8B :5 illustrates that a catalytic converter's efficiency (ability to adsorb and/or desorb oxidants) decreases with its age.

The value of C3 is determined by a mathematical function or map based on the mass airflow in the exhaust manifold 48.

Figure 8C graphically illustrates a preferred mathematical function used in the preferred embodiment of the invention to assign values to C3, depending on the mass airflow rate in the induction manifold 48. As can be seen, 35 the adsorption/desorption efficiency of the catalyst decreases as the mass flow rate increases. The value of C4 is derived from the adaptive parameters (240) calculated by

the oxidant level/capacity controller (232). The C4 value essentially provides feedback capabilities to the model, making the preferred embodiment of the model a closed-loop system. Specifically, the value of C4 is read from a two 5 dimensional look-up table of adaptive parameters. The primary index to the look-up table is air mass flow (202).

For each air mass flow value, there are two C4 values - one for when the catalyst is adsorbing oxidants (equation 3(a)) and one for when the catalyst is desorbing oxidants lo (equation 3(b)). Thus, the value of C4 used in equations 3(a) and 3(b) above varies from time to time with the measured air mass flow in the engine. Further, the values in the C4 lookup table are all adjusted from time to time based on a feedback error term. In particular, the C4 values 15 initially start out as 1. During operation, the estimated oxidant storage level in the catalyst, as determined by this oxidant predictor model now being described, is compared to an oxidant level as measured by oxygen sensors in the catalyst (i.e., sensors 902, 904, 906 in Figure 9) and so outside of the catalyst in the exhaust stream (i.e., sensors 53 and 54 in Figure 1). The difference between the estimated amount of stored oxidants and the measured amount of stored oxidants is considered an oxidant feedback error term. The values in the C4 look-up table are adjusted from 25 time to time based on the oxidant feedback error. A more detailed discussion of the oxidant feedback error and the adjustment to the C4 values is set forth below in connection with the discussion of Figure 7.

so The above-description of applying the feedback

parameter C4 is different if the system does not have oxygen sensors positioned behind each of the bricks, as shown in Figure 9. If such oxygen sensors do not exist, then the system depends only on the feedback signal derived from 35 post-catalyst oxygen sensor 53. Thus, it is not possible to decouple individual adsorbtion/desorbtion rates from the individual bricks. Under these circumstances, a single two

dimensional look-up table (indexed by air mass values) of C4 values is used, and the same C4 parameter is multiplied by the oxidant storage estimate for each brick in the catalyst.

When a single set of C4 parameters are used (as opposed to different C4 values for each brick), it is possible to weight the adsorbtion/desorbtion contributions of the bricks according to pre- determined weighting factors.

In Equation (3a), the value of ka represents the lo maximum adsorbing rate of the catalytic converter in terms of grams of oxidants per second per cubic inch. Similarly, in Equation (3b), the value of kd represents the maximum Resorbing rate of the catalytic converter in terms of grams of oxidants per second per cubic inch. The values of ka and 15 kd are predetermined based on the specifications of the

particular catalytic converter being used.

The value for Max O2 in both Equation (3a) and Equation (3b) represents the maximum amount of oxidants that the So catalyst 52 is capable of storing in terms of grams. This is a constant value that is predetermined according to the specifications of the particular catalytic converter used in

the system. The value for Stored 02 in Equations (3a) and (3b) represents the previously-calculated current amount of s oxidants stored in the catalytic converter 52 in terms of grams. The value for Stored O2 is read from RAM 116.

The value for O2 Flow Rate in Equation (3a) and Equation (3b) represents the mass air flow rate in the induction 30 manifold 18, which is measured by mass air flow sensor 158.

The Base Value in Equation (3a) and Equation (3b) represents the oxygen flow rate where Kd and Ka were determined and it is (PPM 02 of input gas) * (volumetric flow rate) * (density of 02).

The Cat Vol parameter in Equation (3a) and Equation (3b) represents the total volume of the catalytic converter

- 29 in terms of cubic inches. This value is pre-determined based on the type of catalytic converter being used. The value aT in both equations represents the elapsed time in seconds since the last estimation of the change in oxidant s storage in the catalyst.

Finally, the values of N1, N2, Z1, and Z2 are exponents that express the probability of desorption/adsorption and they are determined by experimentally measuring rates of lo adsorption/desorption at given levels of storage and flow.

The exponents are regressed from measurements and can be used to describe linear to sigmoid probabilities.

After the change in estimated oxidant storage in the 15 catalyst 52 is calculated according to Equation (3a) or Equation (3b), the running total of the current oxidant storage maintained in RAM memory 116 is updated accordingly.

Specifically, the amount of oxidants either adsorbed or Resorbed is added/subtracted to the running total of oxidant so storage, which is maintained in RAM memory 116.

The oxidant predictor model may be employed either in an open loop manner or a closed loop manner, as is known to those skilled in the art in view of this disclosure. In an

25 open loop embodiment, the oxidant predictor model described hereinabove estimates the volume of oxidants stored in the catalyst based on various parameters, such as temperature, air mass flow rate, etc., without input from any feedback parameters. Modifying equations 3(a) and 3(b) above to so eliminate the C4 variable would illustrate a preferred open loop embodiment of the oxidant predictor model.

In a closed loop embodiment, on the other hand, the oxidant predictor model further includes a mechanism for 15 adjusting the estimated volume of stored oxidants in the catalyst based on various feedback signals. In particular, after the oxidant predictor model estimates the volume of

oxidants stored in the catalytic converter at a particular time, according to the method described above, this estimated value is used to calculate various other predicted parameters that are compared against corresponding measured 5 feedback parameters. In the preferred embodiment of the invention described above, the C4 variable provides feedback based on the measurements of the catalyst oxygen sensors (i.e., sensors 902, 904, 906) and the pre-catalyst oxygen sensor 54. The feedback parameters could also comprise lo signals from the downstream EGO sensor 53 (shown in Figure 1) or any of several other well-known feedback parameters.

Regardless of the specific feedback signal used, the value of the feedback signal would be compared to the value of the parameter calculated from the estimated oxidant storage level in the catalyst, and the result of the comparison would be a feedback error term. The feedback error term would be used to increase or decrease the estimate of the volume of stored oxidants, as calculated by the oxidant predictor model per the method described above. The so implementation of a closed-loop embodiment of the oxidant predictor model may be advantageous because the feedback signals may enable the oxidant predictor model to more accurately estimate the volume of oxidant stored in the catalyst. In the preferred embodiment of this invention, 25 the C4 parameter, which is adjusted based on the adaptive parameters described in Figure 7, is applied to adjust the oxidant predictor model. Thus, the preferred embodiment of the invention adjusts the predicted level of oxidant stored in the catalyst in a closed-loop fashion.

In the preferred embodiment of the oxidant level predictor, a reset parameter also affects the model. In particular, if the comparison between the estimated amount of stored oxidants and the measured amount of stored 35 oxidants produces a very large oxidant feedback error (i.e., greater than a certain reference value), which may occur as a result of large transients in the system, then it is

desirable to "reset" the oxidant level predictor model instead of allowing the model to gradually correct itself.

For example, if the measured oxidant level in the catalyst is very high, but the estimated oxidant level is very low, 5 then the oxidant level predictor may reset itself to a relatively high storage value. Similarly, if the measured oxidant level in the catalyst is very low, but the estimated oxidant level is very high, then the oxidant level predictor may reset itself to a relatively low storage value. The 10 "reset" function is a second form of corrective feedback in the model, and it facilitates more rapid correction of large errors. Those skilled in the art, in view of this disclosure,

15 will recognize various modifications or additions that can be made to the above-described oxidant predictor model. For example, a well-known heated exhaust gas oxidant (HEGO) sensor, which generally provides an output signal indicative of only a lean or rich condition, can be used in place of so the downstream EGO sensor 53. In this case, when the downstream HEGO sensor provides a signal somewhere between lean and rich, no adjustment is made to the estimated amount of oxidants stored in the catalyst. On the other hand, when the downstream HEGO clearly indicates a lean air/fuel 25 condition, the amount of estimated stored oxidant in the catalyst can be set to the maximum amount that can be stored at the current vehicle operating conditions. Further, when the downstream HEGOsensor indicates a clearly rich air/fuel condition, the estimated amount of stored oxidant can be set so to zero. These adjustments represent a resetting of the estimated amount of oxidants stored based on the downstream HEGO sensor. According to the present invention, the improvement in the estimated amount of oxidants stored in the catalyst 52 based on a feedback error signal can result 35 in improved catalyst emissions.

- 32 Referring now to Figure 7, the oxidant level/capacity controller (232) is described in more detail. A first object of the oxidant level/capacity controller (232) is to calculate an air/fuel control bias for the purpose of 5 adjusting the air/fuel ratio in the engine cylinders to maintain the actual oxidant storage level in the catalyst 52 at or near the oxidant set point. A second object of the oxidant level/capacity controller (232) is to calculate an engine spark delta value and an air mass bias value, both of lo which are used to control the oxidant storage capacity of the catalyst 52 through adjusting the temperature of the catalyst. A final object of the oxidant level/capacity controller (232) is to calculate reset and adaptive parameters based on feedback signals from the oxygen sensors 15 in the exhaust stream and in the catalyst.

The first function of the oxidant level/capacity controller (232) is generally accomplished by comparing the oxidant set point (225) to the estimated actual amount of So oxidants stored in the catalytic converter 52 at a particular time T. The difference between the actual amount of oxidants stored in the catalytic converter 52 and the oxidant set point (225) is referred to herein as the "set point error." The set point error indicates whether the 25 volume of oxidants stored in the catalytic converter 52 is too high or too low relative to the oxidant set point.

Based on the set point error, an air/fuel control bias signal is generated, which affects the ultimate air/fuel control signals sent by the controller 15 to the fuel 30 injectors 18 to adjust the air/fuel ratio either more rich or more lean. Specifically, if the estimated actual amount of oxidants stored in the catalytic converter is less than the oxidant set point, then the controller 15 will adjust the amount of fuel supplied to the engine cylinders so that 35 the engine air/fuel ratio is more lean. On the other hand, if the estimated actual amount of oxidants stored in the catalytic converter is more than the oxidant set point, then

the controller will adjust the amount of fuel supplied to the engine cylinders so that the engine air/fuel ratio is more rich.

5 Referring specifically to Figure 7, the following input parameters are used in connection with determining the air/fuel control bias value: (i) current oxidant storage per brick (231); and (ii) oxidant set point (225). First, at block 711, the estimates of oxidants currently stored in o each of the catalyst bricks (signal 231) are summed, resulting in an estimate of the total amount of oxidants currently stored in all of the bricks of catalyst 52. Next, the set point error is determined by comparing the total oxidants currently stored in the catalyst (711) to the 15 oxidant set point (225) at block 734. The set point error is provided to a proportional-integral controller (blocks 736, 738, and 742), which calculates an air/fuel control bias term. In a preferred embodiment of the invention, the proportional-integral controller uses the set point error to so calculate a closed-loop fuel bias term according to a proportional-integral strategy similar to that described in detail in U.S. Patent No. 5,282,360 to Hamburg, which is hereby incorporated by reference. Specifically, as described in the Hamburg patent, a "window" is defined 25 around the catalytic set point. For example, if the catalytic set point is determined to be X, then the lower limit of the "window" might be set at X-Y and the upper limit of the "window" would be set at X+Z. The variables X and Z represent specific variances from the catalytic set so point.

In relation to the Hamburg patent, the lower and upper limits of the "window" (X-Y) correspond to the rich and lean limits described in the Hamburg patent at lines 1:62-2:5.

35 The upper and lower limits of the window are selectively determined based upon vehicle operating conditions, such as vehicle speed, engine load and engine temperature, as is

known in the art. When the estimated oxidant volume (derived by the observer 206) is outside of the "window", then the commanded air/fuel ratio (provided to the engine cylinders) is linearly ramped so as to urge the oxidant 5 storage in the catalyst toward the oxidant set point. For example, when the estimated oxidant volume is greater than the upper limit of the window, then the commanded air/fuel ratio is linearly ramped in the rich direction, and when the estimated oxidant volume is less than the lower limit of the lo window, then the commanded air/fuel ratio is linearly ramped in the lean direction. When the estimated oxidant volume is between the lower and upper limits of the window, the air/fuel ratio is urged toward the oxidant set point according to a value that is proportional to the difference 15 between the estimated volume of oxidant stored in the catalyst 52 and the oxidant set point. Further details of the preferred proportional-integral air/fuel ratio control strategy are set forth in the Hamburg patent.

so In addition to calculating a proportional-integral fuel bias term, the set point error is also used to schedule an open loop fuel demand value based on the estimated oxidant level in the catalyst. At block 744, the system determines whether to apply the closed-loop proportional-integral fuel Is bias term or the open loop fuel demand, based on various operating parameters, as is known in the art. For example, the open-loop fuel demand parameter may be used in place of the closed-loop fuel bias term in the event of a very large set point error value, indicating irregularities in the so system. The open-loop fuel demand parameter may also be used just after the vehicle has been operated in a deceleration fuel shut-off mode, in which case a period of rich air/fuel operation is required to control the abundance of NOx in the system. Further, the open- loop fuel demand 35 parameter may be used just after the vehicle has been operated according to an open-loop enrichment mode (where fuel is used to keep catalyst temperatures down during high

- 35 load conditions), in which case a period of lean air/fuel operation is desirable to re-oxidize the catalyst and lower hydrocarbon emissions. Whether open loop rich or lean, the magnitude and duration are used to facilitate a rapid return 5 to the 02 set point.

Finally, as shown at block 746, either the closed-loop fuel bias term or the open loop fuel demand parameter is provided to the engine controller 15, which adjusts the fuel lo provided to the engine cylinders based thereon.

The second objective of the oxidant level/capacity controller (232), i.e., oxidant capacity control of the catalyst 52, will now be discussed in more detail.

15 Referring again to Figure 7, the following inputs are used to calculate delta spark and induction air mass bias values: (i) available oxidant storage in each brick (227); (ii) current oxidant storage in each brick (231); (iii) engine spark driveability limits (216); exhaust flange temperature 20 (220); and MET spark (222). First, the estimates of available oxidant storage and current oxidant storage in each of the catalyst bricks are summed (blocks 710 and 711), resulting in an estimate of the total available oxidant storage in the catalyst and an estimate of the total current 25 amount of oxidants stored in the catalyst, respectively.

Then, the total available oxidant storage value (710) is compared to the total current estimated oxidant storage in the catalyst (711) at block 713. At block 702, a spark retard value is calculated based on the difference between So available oxidant storage and current oxidant storage in the catalyst (from block 713) and spark driveability limits (216). In the preferred embodiment of the invention, the spark retard value (702) is read from a look-up table, wherein the values are empirically determined. The spark 15 retard values in the look-up table generally describe the well-known relationship between oxidant storage and brick temperature, as shown in the graph set forth in Figure 8A.

- 36 The spark driveability limits, which are pre-determined inputs to the system, limit the magnitude of the spark retard (702) to ensure that vehicle driveability is not compromised. At block 703, a spark retard gain is calculated based on the exhaust flange temperature (220). Generally, if the flange temperature (220) is relatively high, or increasing, due to high air mass flow or engine air/fuel ratio, then the lo oxidant storage capacity of the catalyst will increase independently of the spark. Thus, a relatively hot flange will permit the catalyst to achieve a desired temperature (and thus oxidant storage capacity) with a relatively lesser delta spark. This is desirable to improve fuel economy. In 15 the preferred embodiment of the invention, the spark retard gain (703) is read from a look-up table, the values of which are empirically- determined. In general, the values in the spark retard gain table follow the graphical function illustrated in Figure 10. The spark retard gain ( 703) is 20 multiplied by the spark retard value (702), as shown at block 704, which results in a delta spark value (728). The delta spark value (728) is provided to the engine controller 15 to adjust the engine spark, and ultimately the oxidant storage capacity of the catalyst.

Generally speaking, the larger the difference between the total available oxidant storage in the catalyst and the total current oxidant storage in the catalyst then the greater the delta spark value.

However, as spark retard increases, engine rpm will fall if not compensated by additional air mass flow through the engine. Accordingly, the delta spark value (728) is used with the MBT spark input value (222) at block 706 to as calculate a required engine torque value, as is known in the art. At block 708, the induction air mass necessary to maintain the required torque is calculated. In the

preferred embodiment of the invention, the desired air mass flow is calculated by dividing the base air mass flow requirements of the engine by an adjustment factor, which is read from a look-up table. The adjustment factors in the s look-up table range from 1, when at MET, to some fractional value down to zero as spark retard increases. Thus, as spark retard increases, the desired air mass flow increases.

This air mass value comprises the air mass bias value (730), which is used by the engine controller 15 to adjust the JO induction air mass in the engine 13. The adjustments to the engine spark and induction air mass adjust the temperature of the exhaust expelled from the engine and thus, ultimately, the temperature of the catalyst 52. Because the oxidant storage capacity of the catalyst 52 depends on its 15 temperature, the engine controller 15 is able to adjust the oxidant storage capacity of the catalyst 52 by adjusting the engine spark and induction air mass flow. This aspect of the invention is particularly useful during certain vehicle operating conditions when the catalyst temperature may fall JO to a level that would otherwise limit the oxidant storage capacity of the catalytic converter 52 to an undesirable small amount. By controlling engine operating conditions to provide a desired catalyst temperature, a certain minimum amount of total oxidant storage capacity can be maintained 25 SO that it is possible to control the actual oxidant storage to a mid-region and prevent break-through of emissions on the lean and rich air/fuel sides.

The third objective of the oxidant level/capacity JO controller is to determine reset/adaptive parameters that are used to adjust the operation of the system on a feed-

back basis.

The reset/adaptive parameters (732) are calculated 35 based on the following inputs: (i) current oxidant storage in each brick (231); (ii) oxygen sensor feedback from each brick (214);

(iii) induction air mass (202); and (iv) measured air/fuel ratio in the exhaust (212).

The feedback signals from the oxygen sensors associated 5 with each of the catalyst bricks (214) (exemplary sensors 902, 904, and 905 shown in Figure 9), which are in terms of voltage levels, are converted to oxidant concentration values at block 712. A similar function is performed at block 716 to convert the feedback signal from the pre o catalyst oxygen sensor 54 located in the exhaust stream to an oxidant concentration value. At block 714, the measured air mass flow rate (202) in the induction passage is integrated over a sample time interval to provide a total air mass in terms of grams. At block 718, a time constant 15 value is determined from a look-up table based on air mass.

The time constant is used to align the pre-catalyst oxygen sensor 54 and the post-catalyst oxygen sensor 53 in time to facilitate an accurate measure of the oxidants that are adsorbed or desorbed in the catalyst.

At block 720, the measured oxidant concentrations of the individual bricks (from block 721) are multiplied by the total air mass in grams (from block 714). The result of block 720 is the amount of oxidants measured at the catalyst 25 brick. Similarly, the time constant determined from the look-up table (block 718) is multiplied by the total air mass (from block 714) at block 722. The result is the amount of oxidants measured in the exhaust stream. At block 724, the results of blocks 720 and 722 are compared, and the so result is integrated over a time constant (in block 725) to give a total measured amount of oxidants in the exhaust stream over the given time period. The final integrated result is the total measured amount of oxidants stored in the catalyst 52. At block 726, the total measured amount of 35 oxidants stored in the catalyst is compared to the estimated amount of oxidants stored in the catalyst (estimated from the oxidant predictor model). The result is an "observer

error." The observer error represents the degree of disagreement between the measured level of oxidant storage in the catalyst and the estimated level of oxidant storage in the catalyst. Based on the observer error, an observer 5 gain is calculated at block 728. The observer gain is used to adjust the two-dimensional look-up table of feedback parameters C4 (described above) that are used to adjust the oxidant level predictor (608). Specifically, at block 730, the observer gain is multiplied by each of the C4 feedback 0 parameters in the two-dimensional look-up table. At block 732, the recalculated two-dimensional look-up table of C4 values is provided to the oxidant level predictor (608) and other algorithms in the system requiring closed-loop feedback adjustments.

Further, a reset parameter is calculated at block 730 based on the magnitude of the oxidant feedback error. If the oxidant feedback error is greater than a certain reference value, then a reset parameter indicative of so resetting the oxidant predictor model (608) to either a low oxidant level or a high oxidant level, as the case may be, is determined.

The description of the preferred embodiment of the

Is invention focuses on a system having one catalytic converter (52). However, the scope of the invention also includes systems comprising multiple upstream and downstream catalytic converters, wherein each of the catalytic converters can have one or more internal catalyst bricks.

So For systems having multiple catalytic converters, the above-

described system would be adapted as now described.

In particular, adaptation of the oxygen storage model from a single brick to multiple brick system is accomplished as by cascading oxygen output from upstream bricks to downstream bricks. The ratio of air to fuel, a measure of excess/deficiency O2 from stoichiometric entering the first

l - - 40 brick is measured or calculated from the fuel control algorithm. Therefore, the excess/deficiency of oxygen can be calculated as described earlier. The amount of oxygen adsorbed/desorbed by the first brick from the exhaust gas is s calculated as described. By adding the oxygen stored or supplied to the exhaust feed gas the post brick a/f, excess/deficiency can be calculated. The second brick O2 storage is then calculated with a similar set of equations, modified for temperature and wash-coat differences. In this lo way output from one brick is cascaded to the following brick. While preferred embodiments of the present invention have been described herein, it is apparent that the basic construction can be altered to provide other embodiments that utilize the processes and compositions of this invention. Therefore, it will be appreciated that the scope of this invention is to be defined by the claims appended hereto rather than by the specific embodiments that have so been presented hereinbefore by way of example.

Claims

1. A method of estimating a change in an amount of oxidants stored in a catalyst of an exhaust system coupled 5 to an internal combustion engine, comprising determining a first amount of oxidants available for being stored in the catalyst and a second amount of oxidants required to oxidize by-products being produced by the engine based on at least an engine air/fuel ratio and estimating a third amount of lo oxidants that are retained by or released from the catalyst over a period based on said first amount of oxidants available for being stored in the catalyst and said second amount of oxidants required to oxidize hydrocarbons being produced by the engine. --

2. A method as claimed in claim 1 wherein said step of estimating said third amount of oxidants that are retained by or released from the catalyst is further based on a flow rate of oxidants in an exhaust manifold and a so storage volume of the catalyst.

3. A method as claimed in a claim 1 or in claim 2 wherein said step of estimating said third amount of oxidants that are retained by or released from the catalyst 25 is further based on a temperature of the catalyst.

4. A method as claimed in any of claims 1 to 3 wherein said step of estimating said third amount of oxidants that are retained by or released from the catalyst So is further based on a parameter indicative of a deterioration of the catalyst.

5. A method as claimed in any of claims 1 to 4 wherein said step of estimating said third amount of 35 oxidants that are retained by or released from the catalyst is further based on a parameter indicative of engine air mass flow.

6. A method as claimed in any of claims 1 to 5 in which the by-products are reductants.

7. A method as claimed in any of claims 1 to 5 5 wherein the by-products are hydrocarbons being produced by the engine and the method further comprises of adjusting said estimate of said third amount of oxidants that are retained in or released from the catalyst based upon a measured operating parameter.

S. A method as claimed in claim 7 wherein said measured operating parameter is determined based on a comparison of an estimated total amount of oxidants stored in the catalyst and a measured amount of oxidants stored in the catalyst.

9. A method as claimed in claim 7 or in claim 8 wherein said measured operating parameter includes a parameter indicative of air mass flow.

10. A method as claimed in any of claims 7 to 9 wherein adjusting said estimate of said third amount of oxidants that are retained in or released from the catalyst further comprises of adjusting said estimate of said third 25 amount to a relatively low amount if said measured amount of oxidants stored in the catalyst is relatively low and adjusting said estimate of said third amount to a relatively high amount if said measured amount of oxidants stored in the catalyst is relatively high.

11. A method as claimed in claim 7 wherein there is an exhaust gas sensor coupled to the exhaust system downstream of the catalyst and the measured operating parameter is the output from the sensor.

12. A method as claimed in claim 11 wherein adjusting said estimate of said third amount of oxidants that are

À 43 retained in or released from the catalyst further comprises of setting said estimate of said third amount of oxidants to a relatively low value when said sensor output indicates a mixture rich of stoichiometric and setting said estimate of said third amount of oxidants to a relatively high value when said sensor output indicates a mixture lean of stoichiometric.

13. A system for deriving an air/fuel adjustment lo control signal for an internal combustion engine coupled to an exhaust system having a catalyst, the system comprising an oxidant set point generator that is responsive to at least one engine operating parameter and a set point location signal to determine a target oxidant level in the 15 catalyst, a current oxidant level estimator that is responsive to at least one engine operating parameter and that estimates a current amount of oxidants stored in the catalyst, said current oxidant level estimator selectively determining an amount of oxidants available for being stored To in the catalyst and an amount of oxidants required to oxidize hydrocarbons being produced by the engine based on an engine air/fuel ratio and an oxidant level/capacity controller that is responsive to said target oxidant level and said current amount of oxidants stored in the catalyst, 25 and that determines an air/fuel adjustment control signal.

14. A system as claimed in claim 13 wherein said current oxidant level estimator further estimates a volume of oxidants that are adsorbed in or desorbed from the so catalyst over a period of time based on said amount of oxidants available for being stored in the catalyst and said amount of oxidants required to oxidize hydrocarbons being produced by the engine.

15 15. A system as claimed in claim 13 or in claim 14 wherein said current oxidant level estimator further estimates said volume of oxidants that are adsorbed in or

- 44 desorbed from the catalyst based on a flow rate of oxidants in an exhaust manifold and a volume of the catalyst.

16. A system as claimed in claim 13 or in claim 14 5 wherein said current oxidant level estimator further selectively estimates a volume of oxidants that are adsorbed in or Resorbed from the catalyst based on a temperature of the catalyst.

lo

17. A system as claimed in claim 13 or in claim 14 wherein said current oxidant level estimator further estimates a volume of oxidants that are adsorbed in or desorbed from the catalyst based on a parameter indicative of a deterioration of the catalyst.

18. A system as claimed in claim 13 or in claim 14 wherein said current oxidant level estimator further estimates a volume of oxidants that are adsorbed in or desorbed from the catalyst based on a parameter indicative so of engine air mass flow.

19. A method substantially as described herein with reference to the accompanying drawing.

20. A system substantially as described herein with reference to the accompanying drawing.